This invention relates to a stacking wheel for stacking sheets and to a sheet-goods processing apparatus comprising such a stacking wheel, in particular for processing value documents.
Stacking wheels of the above-mentioned kind possess sheet slots distributed over the circumference for receiving sheets of the sheet goods to be stacked. The sheet slots extend along a substantially spiral-shaped course from radially outward to radially inward within the stacking wheel. Hence, sheet-goods processing apparatuses equipped with such stacking wheels are also designated spiral slot stackers.
Automatic sorting machines in which sheet-shaped articles are sorted are to be able to process maximum numbers of pieces within a minimum time, which necessarily leads to high transport speeds. In conventional spiral slot stackers it is problematic to increase the transport speed to more than 40 sheets per second, since the sheets running into the stacking wheel must be decelerated to a standstill on a short path within a short time. In particular, the sheets can thereby hit the end of the sheet slots and consequently be damaged at the leading sheet edges.
DE 3232348 A1 describes a spiral slot stacker that has a plurality of stacking-wheel disks rotating around a common axle and having sheet slots extending from the outside to the inside in a spiral shape. The sheet slots of the disks each form a delivery slot into which a sheet is inserted tangentially. Using a stripper disposed between the disks the sheets are stripped out of the delivery slots. The circumferential speed of the disks is substantially lower than the transport speed of the sheets, so that the sheet running in slides with its surface frictionally along the outer boundary walls of the spiral-shaped sheet slots. The frictional force resulting from the relative motion decelerates the sheet. To increase the friction for decelerating the sheets running into the stacking wheel, it is proposed in DE 3232348 A1 to mutually “stagger” the spiral-shaped sheet slots of the stacking-wheel disks disposed side by side on the common axle. Staggering is to be understood to mean that the spirally extending sheet slots are not mutually congruent when viewed along the drive axle, but extend differently. The sheets running into the sheet slots thereby become wavy in a direction transverse to the moving direction of the sheets. This can be obtained e.g. with identical stacking-wheel disks whose sheet slots all have the same spiral curvature when at least one of the stacking-wheel disks is mounted on the drive axle so as to be rotated by a small angular amount relative to the other stacking-wheel disks. Due to the wavy curvature imposed on the sheets when the sheets are moved along the sheet slots, the pressure of the sheets against the boundary walls of the spiral slots is increased, so that frictional forces and thus the braking action increase accordingly. Upon a further increase of the transport speed of the sheets to be stacked, however, the stacked sheets show damage to the leading sheet edges.